Every second of every day, your body makes damaging free radicals. And every second your body makes a powerful substance called glutathione (abbreviated GSH) that grabs hold of those free radicals and smothers them. Glutathione also picks up any toxic substances (from air pollution, say) that have found their way into your body and escorts them out.
Glutathione is a tripeptide—a small protein made from just three amino acids. Molecules of cysteine, glycine, and glutamic acid combine in your cells to make glutathione.
Of the three aminos needed to make glutathione, cysteine is the most important, because cysteine contains sulfur, which is also needed to make glutathione. You also need the trace mineral selenium to make glutathione. Your body generally has plenty of glycine and glutamic acid (or its close cousin glutamine), but sometimes the cysteine and selenium are in short supply. When that happens, you could end up without enough glutathione to defend your body.